Technical Field
The invention relates generally to apparatus and methods for drilling generally horizontal boreholes. Compressed air may be used to facilitate removal of the cuttings or spoil from the borehole, and a reduced diameter casing may be used to drive rotation of a cutting head.
Background Information
Underground boring machines have been used for decades in the drilling of generally horizontal boreholes, which may include boreholes which are substantially straight and those which are arcuate for the primary purpose of avoiding or bypassing an obstacle. Often such boreholes are formed by initially drilling or otherwise forming a pilot hole of a generally smaller diameter, followed by the use of an enlarged cutting head which follows the path of the pilot hole in order to enlarge the borehole. In some cases, it may take only one pass in addition to the pilot hole to create the desired final diameter of the borehole. In other cases, additional enlarged cutting devices may be used to drill as many passes as necessary to achieve the desired diameter of the borehole.
Many of the boring machines utilize an auger which is rotated in order to force the cuttings or spoil to be removed from the borehole. Such augers may be disposed in a casing and have an outer diameter which is slightly smaller than that of the inner diameter of the casing in which it is disposed. Drilling fluid or mud is often pumped into the borehole either within a casing or external to a casing in order to facilitate the cutting process and removal of the cuttings. Drilling fluids or lubricants may involve water, bentonite or various types of polymers, etc. The use of certain types of drilling fluids may present environmental hazards and may be prohibited by environmental laws or regulations in certain circumstances. The inadvertent return of drilling lubricant, sometimes referred to as “frac-out”, may be of concern when the drilling occurs, for example, under sensitive habitats or waterways. Although bentonite is non-toxic, the use of a bentonite slurry may be harmful to, for example, aquatic plants and fish and their eggs, which may be smothered by the fine bentonite particles when discharged into waterways.
As noted above, many underground boring systems utilize augers to remove the cuttings from the borehole. Such augers are typically formed in sections, which are sequentially added rearwardly as the borehole becomes longer and can accommodate additional auger sections. Given that many boreholes may be several hundred feet long, an auger of such length adds a substantial amount of weight and frictional resistance to the rotation thereof. There is a need in the art for improvements with respect to the above-noted problems.